1. Field of the Invention
The present invention relates to an optical circulator used in optical communications.
2. Prior Art
Non-reciprocal circuits have been used for some time as isolators and circulators, to construct a microwave circuit in microwave regions. This non-reciprocal circuit was introduced theoretically in 1984, its first realization being a microwave gyrator developed in 1952. Thereafter, research has led to a variety of application developments. The above isolator has been used either to isolate oscillators from loads so as to offer smooth operation, or to remove acoustic distortion in signal waves transmitted over a long distance of transmission path. The circulator has also been used in transmitting and receiving common circuits and partial wave circuits.
Such a non-reciprocal circuit can naturally be used in the area of optical wave communications as in microwaves. For instance, an optical isolator has already been in use to prevent noise caused by light returning from a reflecting point back to oscillation sources or optic amplifiers.
A specific example of the above optical isolator is given in FIGS. 1(A) and (B). In this optical isolator, a light injected from a first light incoming and outgoing port consisting of an optical fiber 201 converges and forms an image through a lens 202, and passes through a flat double refraction crystal 203. As a result of passing through the flat double refraction crystal 203, an extraordinary light is displaced, and the extraordinary light and ordinary light proceed on different paths and enter a magnetic optic material 204. Then, the extraordinary and ordinary lights rotate clockwise as much as 90.degree. at the magnetic optic material 204, and a crystal 205, which has optical rotary power or anisotropy, and are injected into the double refraction crystal 206. The extraordinary light is displaced again when passing through the double refraction crystal 206, hence the polarization center points of both lights finally coincide, and enter the second incoming and outgoing port consisting of an optical fiber 207 without a loss.
Next, an explanation is given on a light that proceeds from the optical fiber 207 to an optical fiber 201. Because the magnetic optic material 204 has directionality, it is rotated for a change of as much as 45.degree. counterclockwise relative to the proceeding direction of the light in this case. Therefore, the polarization centers of the extraordinary and ordinary lights neither coincide nor enter the optical fiber. In the meantime, the detail of this isolator is disclosed in Patent Publication No. Sho-58-28561.
In addition, the optical circulator, though not having reached a stage of practical use, may be applied in microwave applications, and is likely to be used as an optical circulator now that optic amplifiers have become practical.
For specific examples of such optical circulators, optical circulators 1 are proposed as shown in FIGS. 1 and 2.
Such optical circulators 1 are shown, for example, in the following theses: T. Matsumoto et al, "Polarization-independent optical circulator: an experiment, Appl. Opt. Vol. 19, No. 1, pp. 108-112, 1980; and M. Koga et al., "Multi-Demultiplexer Using a 4-port Optical Circulator and Interference Filters", The Trans. IEICE of Japan, Vol. E72, No. 10, pp. 1086-1088, 1989. These circulators comprise polarization beam splitters 2 and 3; angular prisms 4 and 5; 45.degree. quartz rotators 6 and 7; 45 Faraday rotators 8 and 9; and light incoming and outgoing ports 11 through 14.
In the optical circulator 1, a light Li injected from a light incoming and outgoing port 11 is divided into a P-wave content (L1) and an S-wave content (L2) by a polarization beam splitter 2, as shown in FIG. 2. The P-wave content (L1), as a result of transmitting through the 45.degree. quartz rotator 6 and 45.degree. Faraday rotator 8, has its electrical field vibration direction rotated by 90.degree., is reflected by the angular prism 5, and injected into the polarization beam splitter 3. The S-wave content (L2), as a result of being reflected by the angular prism 4 and transmitting through the 45.degree. quartz rotator 7 and 45.degree. Faraday rotator 9, has its electrical field vibration direction rotated by 90.degree., and is injected into the polarization beam splitter 3. The P-wave content (L1) and S-wave content (L2) are combined by the polarization beam splitter 3, and the combined light Lf goes out from the light incoming and outgoing port 12.
Also as shown in FIG. 3, the light Li injected from a light incoming and outgoing port 12 is divided into a P-wave content (L3) and an S-wave content (L4) by a polarization beam splitter 3. The P-wave content (L3) transmits through the 45.degree. Faraday rotator 9 and 45.degree. quartz rotator 7, and is then reflected by the angular prism 4 and injected into the polarization beam splitter 2. On the one hand, the S-wave content (L4) is reflected by the angular prism 5, and transmits through the 45.degree. Faraday rotator 8 and 45.degree. quartz rotator 6, and is injected into the polarization beam splitter 2. The P-wave content (L3) and S-wave content (L4) are combined by the polarization beam splitter 2, and the combined light Lf passes out from the light incoming and outgoing port 13. In this case, the light directions of the P-wave content (L3) and S-wave content (L4) are opposite to those of the said P-wave content (L1) and S-wave content (L2), so the electrical field vibration directions will not rotate even when passing through the two reciprocal and non-reciprocal rotators.
As explained above, the optical circulator 1 can perform a non-reciprocal operation such that the light Li injected from the light incoming and outgoing port 11 passes out from the light incoming and outgoing port 12, and the light Li injected from the light incoming and outgoing port 12 passes out from the light incoming and outgoing port 13 rather than from the light incoming and outgoing port 11.
The above optical isolator normally has two light incoming and outgoing ports, wherein the light injected from the first light incoming and outgoing port passes out from the second light incoming and outgoing port. However, the light conversely injected from the second light incoming and outgoing port will not pass out from the first light incoming and outgoing port. Therefore, it is very difficult to save and utilize the light injected from the second light incoming and outgoing port, as bidirectional communication medium without wasting.
In addition, the optical circulator 1 has an advantage of effectively utilizing the light Li injected from the light incoming and outgoing port 12, as bidirectional communication medium by means of leading the light to the incoming and outgoing port 13, but at the same time has a drawback in that, because the optical circulator 1 is basically constructed of the polarization beam splitters 2 and 3 and angular prisms 4 and 5 as explained above, the isolation cannot be increased when the polarization beam splitters 2 and 3 are used. The reason for this is that it is technically difficult to suppress to 30 dB or lower the isolation quantity of P-wave content to be transmitted that leaks into the S-wave content to be reflected by the polarization beam splitters 2, 3.
Furthermore, when the angular prisms 4 and 5 are used, there is a disadvantage in that high-level technology is required to optically link the incoming and outgoing port 11 with 12 (11 with 13). For instance, the S-wave content (L1) and P-wave content (L2) that have been divided by the polarization beam splitter 2 must have its direction changed by the angular prisms 4 and 5, and again must be combined by the polarization beam splitter 3. Therefore, the angular prisms 4 and 5 must be mounted upon adjusting their angles at a high accuracy so that the proceeding directions of the combined S-wave content and P-wave content may be coincided as close as possible. Since these angles need be adjusted in a second-order accuracy in the case of, for example, the incoming and outgoing ports 11, . . . in single mode wave guide path, the angle at the reflecting face must provide second-order accuracy in fabricating the angular prisms 4 and 5. Thus, the optical circulator 1 has a disadvantage in that it requires a very high level optical linking technique.